In recent years, in view of an increasing demand for traffic noise reduction, there has been a demand for reducing intake noises generated at an air intake portion of a vehicle engine. At the air intake portion of the engine, an air intake duct is provided. The intake noises are amplified by pipe resonance generated inside the air intake duct, and are radiated from an air intake port. Therefore, conventionally, in order to reduce the intake noises, a “resonant type muffler” called as a resonator or a side branch is provided.
However, in an engine room, a number of devices such as various auxiliary devices of the engine and electronic devices for reducing exhaust gas are provided. Therefore, available space inside the engine room is limited, and it is difficult to provide a space for installing the resonant type muffler. In view of this, Patent Document 1 proposes an air intake duct having openings in the middle of a duct body, the openings being covered with a sheet member of heat-shrinkable fibers to reduce intake noises originating from pipe resonance.
Patent Document 1: JP 2000-282985 A